text:
Flying in a helicopter with open doors: why don't people fall out?
This has been puzzling for quite a long time.
Every action movie or military documentary features helicopters with no door or with open doors and yet people inside look like they couldn't care less. Apparently they're not falling out...
Why is it so?

uhm, last time I checked gravity was still working in the up/down direction, not sidewise.

@Federico Yet I'm sure you know what I meant ;)

uhm, no I don't

Helicopters have belts and straps, even open ones.

Do you mean in a turn?

@CedricH. - ever driven a car with your windows open, top down or, something with detachable (or no doors) like some of the old school jeeps? Did it seem like you were in danger of falling/getting sucked out? Same thing applies to flying helicopeter/airplanes with doors and/or windows open.

My bicycle and my motorbike don't have doors either yet I feel perfectly comfortable riding them. Maybe doors aren't needed to prevent you from falling off / out

@DeltaLima But you're the one driving in that case. OK the pilot is safe, but what about people in the back?

@GdD Maybe that's the answer...

@CedricH.Do you mean in a turn?

@Simon I understand that if it's a nice turn you can just sit in the back and you're good. But it seems that the risk when/if the flights becomes more unpredictable or erratic is quite high..

The same physics apply, no matter how "unpredictable" the flight is - not that I even know what that means.  You would have to try really hard to fall out of a helicopter.

@DeltaLima I'm never going to sit behind you on a motorbike, where it is not safe.

@DeltaLima I'd be quite scared cycling 100 metre above the ground...

How is the force on the pilot different than the force on the passenger?

@Blam You're in the back of a car, doors open, the driver turns suddenly. If  that situation was similar to what people would feel in a helicopter, I would find flying with the doors open extremely dangerous. So I get that it's not the case and I found the accepted answer quite convincing.

cars don't lean into the turn so the turn vector forces the passenger towards the door. in a heli or a motorcycle, the passenger is forced into the seat.

From the action/cinema perspective, some game i've played recentlyish (metal gear maybe) actually had your character clip in and out of some kind of seatbelt/harness/safty strap when switching which side you were hanging out of. No idea how realistic, but it was a nice touch that made me think about these things.

@DeltaLima: You possible weren't riding your bike over bumps.  There've been a number of times when I would have been off if I hadn't been holding on to handlebars - and a few times when I have come off.  I've also been in a light plane and encountered turbulence strong enough to knock my head against the ceiling, despite being belted in.  (I'm fairly tall, so there's only an inch or two clearance anyway.)

People regularly fall off bicycles, out of cars, off boats. People even get blown off park benches on windy days. People who ask questions don't deserve to face a stream of snide and fatuous comments.

Aircraft, fixed-wing or rotorcraft, have limited capability to produce a sideways force, so they simply can't throw you out.
It is a matter of physics. Inertial forces (gravity and centrifugal force) act on you and the aircraft the same, so they won't make you move relative to the aircraft. The only force between you and the seat is caused by the aerodynamic forces. And the wing or rotor can only produce significant force vertically, and some torque with help of control surfaces. But for lateral force there is only drag of the fuselage and the vertical stabilizer and, in most helicopters, anti-torque rotor, which don't produce that much force unless there is really bad turbulence or in some cases if the handling is really bad.
When aircraft turns, it banks to be able to produce the necessary side force, so the resulting force is still mostly straight to the floor.
Of course the other reason is that all aircraft and helicopters have seat-belts and the people sitting by open door usually have them on—except for skydivers, who have the parachute on already, and rescue divers who are tied to the winch instead.

So basically it is harder for my dog to stay stable when he's in the back of my car than it would be for someone in a helicopter?

That's not completely true, you can fall out of a helicopter if the handling is rough enough. It's happened to soldiers before.

@CedricH.: Most of the time, yes.

as a side note and to be precise, the gravitational force is not an inertial one, at least in the context of a helicopter (it can be assimilated to an inertial force in relativity)

That seems very plausible Cederic H. The road can apply a lot of sideways force to your car in a turn through friction with the tyres. There's nothing equivalent to push a helicopter sideways.

@CedricH. If your dog were sitting in a seat like a human with contact between his butt and back and the seat, he'd be a lot more stable, hopefully your seated passengers are not falling out of their seats when you drive, even if they were not buckled in.

@JanHudec In the end this is very convincing argument.

It's also worth noting that a human sitting correctly in a seat (with their back against the backrest) has a fair amount of contact with that seat and so is held, in part, in place by friction.

@GdD That's just the aircraft moving out from under them with their frictional force not strong enough to bring them along. Same principle behind pulling a table cloth off a set table.

@corsiKa I wouldn't say "just"...

@corsiKa, I am sure it would be warm consolation to a soldier as he/she falls out of the helicopter that is was lack of friction that was the culprit.

@CedricH.  A 4 wheel car on pavement is not the same.  Try this, Fill a paint bucket 1/2 way and tie a 4 foot rope to the handle.  Holding the rope only  from 4 feet try and pour out the water.  You can even loop the bucket and not lose water.

@Blam Indeed, that's why I said that the most convincing argument in this whole question is the one given Jan's answer (3rd paragraph).

@CedricH. Then why is the first paragraph about car?

@Blam What first paragraph?

@CedricH. OK first comment

@Blam OK, understand why I asked the question: I'm driving with my dog in the back, that's quite uncomfortable for him, then I wondered "it must be so dangerous in a helicopter". Now I read Jan's answer again and read "mostly straight to the floor" while I thought he said "perpendicular to the floor", which is I think what he meant.

@CedricH. you couldn't ... I wouldn't recommend a unicycle balancing act in the back of a car or helicopter.

@Blam: The comparison to cars is a good one, because who hasn't been sitting in the back seat of a car (foolishly not wearing a seatbelt) and been slid clear across from one side to the other as the driver makes a sharp turn, or even just goes around a particularly tight curve at highway speeds? This answer is good because it explains how "the rules" are different with helicopters (or all aircraft, really), and why the same thing doesn't happen in them.

ie : pilots have these https://en.wikipedia.org/wiki/Turn_and_slip_indicator -- and use them.

Also helicopters don't stop and start like a car does. If a helicopter came to an abrupt halt everyone would fly forward with the inertia.

The reason that the occupants are so calm about being in an open helicopter are because they are strapped in. There are many configurations that open helicopters can have, some with seats and some not. The seats are generally removable, connecting to points on the floor. Occupants in the back have 2 options for strapping in, the first is your typical seat belt, the other is known as a "monkey harness", which is a harness that goes around your shoulders and waist and can be clipped to points in the floor. Door gunners usually preferred harnesses because it allowed much more freedom of movement so they could fire at more angles.
Remember also that these people were used to it as well. I'm sure most of them were bricking it big-time their first few trips no matter how well they were strapped in. Also, I suppose when you're about to be shot at falling out may frighten you less than it may otherwise.

@Cedric Everything looks easier on TV than in real life :)

I taught sport skydiving for 10 years, and I have sat half-out of the open door of an aircraft, no seatbelt, hundreds of times while the aircraft makes 45 degree banks toward the open door side. It's a bit odd seeing the airport above the wingtip.
Unless the plane is in a spin or otherwise spectacularly out of control, the forces are largely perpendicular to the floor of the aircraft. At no time have I ever felt like I was going to fall out, and actually getting out is difficult enough that we spend an hour or more practicing it on the ground.
You asked about helicopters specifically - they are less able to produce cross-fuselage forces than fixed-wing planes. Look at the rotor, then the thin post connecting the rotor hub to the fuselage. It's not that thick and thus cannot produce enough force to toss someone out the door.
Heavy turbulence could bounce you around to the point where falling out is possible. Not guaranteed, just possible if you bounce in that direction. But at low speeds and clear weather it's unlikely to the point where sitting down is adequate. Standing and moving around with an open door is common, but the 1 inch strap that is plenty strong enough to hold you up is very hard to see, especially on TV.

@SteveJessop sounds like Tyler flies in a turbulent area, and/or needs to work on his technique (he certainly needs to work on his harness - they do NOT "pop open"). We used Cessnas in the prairies.

@SteveJessop Tyler apprears to be a fixed wing pilot who does not recognise that helicopters are very different.  I have heard it said on this site several times that in flight, "helicopters fly just like fixed wings". They don't.  Many people think that because you can generate a certain force in a fixed wing, that you must therefore be able to generate the same force in a fixed wing.  This is a fallacy and does nothing to help discussions such as this one.

Consider for a moment what would have to happen for you to fall out. That would have to happen either through gravity, or inertia.
Some rough over-the-thumb rationalizing (and keep in mind that I'm not a pilot, let alone a helicopter pilot -- just some basic physics applying here):
Gravity. The helicopter would have bank at a significant angle, and at the same time not have enough centripetal force (which would press you into your seat via inertia, as during a "normal" bank). For example, if the pilot forces the copter into a bank and then zeroes out on the collective. However, that same gravity will act on the chopper as well... uncomfortable low-g conditions, yes, and you probably would want to grab hold of something, but you don't get ripped out of your seat immediately.
Inertia. The helicopter would have to make a sudden lateral movement, so you get jerked out of your seat. However, as others have pointed out already, a helicopter does not have a way to induce sudden lateral movements of that magnitude. So, no dice.
Once we start considering outside forces -- turbulence, high winds, blasts from ordonance fired at you -- you probably had enough sense to strap yourself in.
Many of the movies you've been seeing will be picturing helicopters operating in Vietnam, Iraq, Afghanistan, Somalia... see a pattern here? Those are hot climates we're talking about. So, given that the usual chopper ride is unlikely to toss you out of the door, why shouldn't people enjoy a bit of a breeze? ;-)

There's another physical aspect that can cause people to fall out: pressure loss in an aircraft with a pressurised cabin.

If you're flying in a chopper with the doors open, @gerrit then you've already lost pressurization...

The point most people don't get, and it's simple physics, is that in a turn, you are constantly accelerating.  The opposing force to that acceleration is what keeps you in place.

@FreeMan What I'm getting is at is if people expect to be blown out just because the door opens, that's probably because they are thinking of spectacular pictures of passenger airlines where the door suddenly blows open.

@gerrit: At the altitudes helicopters are usually operating, decompression is not an issue. (I don't know if helicopters even can achieve altitudes where it would become a problem...)

@DevSolar I know.

I don't fly helicopters, but I can tell you that in fixed wing aircraft you always strap in. I assume helicopters are the same.
If a plane has no doors and you are not strapped in, you will have an excellent chance of falling out. I would assume the same is true for helicopters. Some of the other answerers have talked about centrifugal forces or whatever. In reality what happens is that gusts hit you and knock you around all the time. You don't notice the gusts in big aircraft like Airbuses, but in a small aircraft it can be like being inside of a barrel rolling down a mountain. You definitely do not want to be unsecured at any time in flight, especially with a door open.
Once I was getting towed in a glider and my harness popped open. In about 2 seconds my left foot was sticking straight up, wedged against the bubble canopy and my right knee was in my face. I was upside down. Meanwhile, the glider is moving around like Pirates of the Caribbean and the tow pilot is wondering, "What the hell is he doing?" I had to wedge myself against the sides of the glider and instrument panel like Jackie Chan to get the harness buckled again.
Of course, the knuckleball motion of a gusted aircraft can be an advantage sometimes, such as when you need to deal with an unruly passenger. Just unbuckle his harness when he isn't looking, kick his door open and he will be singing "Free Falling" by Tom Petty in no time :-) Just kidding, of course, don't do that.

I have flown helicopters often with doors off, and although I strap in, it has nothing to do with me "falling" from the aircraft. I have never felt significant force other than perpendicular to the Z axis. Indeed, it's quite possible to roll on 45 degrees of bank and lean out of the door. I would need to shove off the aircraft to "fall out". You say "centrifugal force or whatever" as if it doesn't exist. It does. It's the apparent (or fictitious) force in a rotating frame which opposes centripetal force.  The craft experiences little lateral resistance so does not produce lateral force.

What on earth was going on it that glider, in a tow, to generate the amount of G you describe?

@Simon Try doing an aero tow on a gusty day in a two-seater without wearing a harness. Might be an educational experience for you.

@Simon I don't know about a helicopter, but just slamming on the rudder pedals to perform an unbalanced turn produces quite a sideways force in my experience!

@romkyns Well, the OP asked about helicopters, the discussion is about helicopters and helicopters don't have rudder pedals :)

@Simon Well they do have pedals :) Are you saying you wouldn't fall out if the pilot slammed on a pedal while going at full speed? I think your experience is only suggestive of the fact that your pilot was flying with your precarious positioning in mind.

@romkyns I was the pilot, often with doors off. There is nothing precarious about it. Slamming in a pedal at full speed will kill everyone on board anyway in the resulting crash.  Helicopters do not fly, nor can they be handled, like fixed wing aircraft.  What I'm saying is, that in all normal, and aggressive manoeuvring, you cannot "fall" out of a helicopter, whether strapped in or not.  You cannot defeat physics, nor the characteristics of helicopters.  No matter what you do, to induce that kind of lateral force, you are going to do major damage and in all probability, kill all onboard.

@Simon OK, alright, I'm going to trust you there. A little surprised though, as it would be trivial in an airplane to throw myself right through the plexiglass if I didn't have the harness on in a maneuvre that's only moderately violent and doesn't come anywhere close to causing a crash.

@romkyns What fixed wing pilots really struggle with is how a helicopter works.  The fuselage hangs under the rotor like a pendulum.  It takes a long time for it to move in response to the disc moving and it moves much more slowly and heavily damped. To produce that kind of violence would likely tear off the rotor or chop off the tail before you induced enough lateral force to eject someone.

The axis of rotation for the directional pedals in a tail-rotor helicopter is the main rotor shaft. When you make a 360, you are forced forward (in the front seat), not sideways.

You don't fall, even in the turns because of the the direction of the body acceleration is toward the helicopter floor, including when the pilot turns. So the body is maintained with a force towards the floor, never towards the open door.
On my first ride out to the jungle (Vietnam), I looked for a seat. There weren't any. I was instructed to just sit on the side with feet hanging out. The first ride was interesting, but they soon became routine. The pilot could turn sideways and this force would hold you in. Sometimes we flew so low I thought my feet were touching the treetops.
I think about this 40 some years later. Would I sit on the side again without being strapped in ? I don't think so.

That's what I meant by saying the force would hold you in. I am assuming centrifugal force held you in on turns and gravity in level flight.

To fall out of a helicopter, your center of gravity would have to lie outside of the aircraft.  Because helicopters tend to bank smoothly when they accelerate, your center of gravity stays within the aircraft.  Unless the aircraft were struck by another object, it is unlikely you would ever experience a significant lateral force while flying in a helicopter.

LOL, try this: ask your passenger to take off his harness, open the door and stand in front of the door, then do a heavy, sudden skid to the side opposite the door and see what happens to your passenger's "center of gravity".

I think the ultimate answer lies in the fact that helicopters banks when turning and the resulting force is not purely lateral (as would happen in a car).

@TylerDurden You are a self-confessed "non helicopter driver".  I am a helicopter driver.  it is not possible in a helicopter to generate significant lateral force no matter what you do.  the fuselage is effectively a pendulum hanging beneath the head.  If you make a rapid lateral control input, the disc will respond quickly, and violently, but the fuselage experiences a much delayed, much smaller and highly damped reaction.  I have been in military helicopters being flown as aggressively as it gets. Manoeuvres with that kind of violence would almost certainly result in catastrophic damage.

I'd be curious to see accelerometer/axis logs for various helicopter maneuvers, and see just how much lateral force an occupant actually experiences.  With regards to Tyler's hypothetical, the scenario he presents is one where the occupant is deliberately placing their center of gravity in a precarious location, and then deliberately accelerating the vehicle in a direction intended to push the occupant's cog outside of the vehicle.  I didn't say it was impossible to fall out of a helicopter, just that it is unlikely, for the reasons I provided.

@ByronJones  The seat of my pants is a pretty good accelerometer and my brain a fairly decent log.  I've been in military helicopters in combat manoeuvring and in "normal" helicopters doing aggressive manoeuvring (torque turns, falling leaf, rapid reversals) and so on.  In none of that experience did my accelerometer log significant lateral force nor did I ever get forced towards a door.  The forces are almost entirely vertical pushing you into or pulling you up from your seat.

The helicopter is basically suspended from its main rotor, so you are sitting in the equivalent of a large swing/ride and pulled towards its floor.  Suspended monorails with the ability to lean into curves can likewise look precariously wobbly from the outside while appearing perfectly stable for the inside passengers who don't feel the outside forces exactly because the craft is able to follow them, compensating their effect on the load.
The amount of jerk you can put on a heli by controlling the main blades is limited.  You'll probably cause more problems by switching on and off auxiliary means of propulsion.  However, those don't make for the dramatic dives, banks and turns in movies.

I like the analogy of suspended monorails.  It helps to explain the pendulum effect of the fuselage.

If you mean when the helicopter banks then all the people are either strapped into their seats like your car seat belt as all the people above have mentioned or because the have a cord attached to their jacket. (The cord part is what I saw in a documentary on the Discovery Channel).

You seem to be implicitly comparing to cornering in a car. In a car, turns are achieved my the tyres applying a horizontal force to the road, so you feel an opposite (centrifugal) force pushing you outwards. However, aircraft turn by banking. In a banked into the turn, the forces felt by the passengers are primarily towards the floor.

Actually seat belts in helicopters and airplanes aren't really to keep you in the plane, rather, they are to keep you from losing control if turbulence happens to throw you against the ceiling and/or knock you out. Helicopters usually avoid negative G forces as it does bad things to the rotor system, so becoming airborne is less likely, but still possible. These forces could cause you to levitate and if the turbulence is also of a sidewards component, the helicopter could travel sideways while you are airborne. Guess what?

Why was this downvoted?  There is nothing incorrect in this answer.

Are you expecting them to fly out of the helicopter because in movies, when the plane/spaceship/whatever door is opened, people get sucked out?

(source)
If so, that effect is real (though perhaps exaggerated), but it's not caused by the craft's movement.  It's caused by the depressurization of the room.
The inside has high pressure, while the outside has low pressure.  When a door opens, the high pressure inside-air moves towards the outside, exerting a force on anything it meets (in the same way that the wind pulls you during a nasty storm).  However, the effect dies down quickly as the pressure equalizes.
In a helicopter with open sides, that pressurization never happens to begin with, so there is no depressurization to pull the passengers out.

And what about the high speed relative wind?

@Mageek Just like in a car, the relative-wind caused by the movement of the helicopter is parallel to that movement, so unless you're sticking your head out the door you'd feel very little of it on the inside.

@Mageek.  You never feel the relative wind inside.  By definition, it's impossible since it is relative to the direction of flight.

@Simon But maybe the turbulence?

This doesn't answer the question which state a context of "Flying in a helicopter with open doors"

@mins: The effect OP is expecting does not exist.  I explained why OP is expecting it to, and why it actually doesn't.  What more could I possibly add to answer the question?

No the OP doesn't indicate anything about opening the door quickly in flight. In addition a helicopter usually flies close to the ground where pressure differential would not exist. The OP just wonders why being on an instable vehicle with door open seems not bother occupants.

Might not be relevant but here in the UK, I have been in helicopters on Search and Rescue training and missions in the mountains and you will always be belted in, whether it is by ground entry or winched up. The winchman will be attached by leash and harness as he is usually hanging around or out of the door.

As an "Airmobile" infantryman (101st), I rode in Blackhawks countless times, sometimes in a seat belted in, but usually not.  Maybe because we were only in the Blackhawk to get out of it -- rappelling, fast-roping, helocasting. I don't know if we ever had the doors closed -- can't think of a time, anyway.  I have thought of this before, looking back, and wondered why we never seemed concerned about sitting in the door with our legs dangling out.  I'm guessing that the scientific explanations offered above are the reason why.  But I can tell you, it's not just Hollywood.  We really did sit in or around the open door and never seemed to think twice about it.

While this is good anecdotal information, it is more of a comment than an answer.

People aren't falling because the cabin isn't pressurised.  If you would try to open the door in mid-air of your average passenger æroplane cruising in the upper troposphere, you would be not amused.  People would be sucked out.  But helicopters usually don't fly at altitudes where pressure is a problem, so there is no cabin pressurisation, so there is no outward force due to an open door.

If you tried to open the door of a passenger airliner during cruise, you'd be unamused because you'd be physically unable to do so (the pressure differential holds the door closed) and because you'd be restrained by your fellow passengers and the cabin crew, and arrested on landing.

Take a bucket of water with a handle. Start swinging it in a pendulum motion until it has enough swing to actually go around at the same constant speed. Not a drop of water will be spilled. #Pendulumeffect

please consider expanding your answer

Sitting in a helicopter in no way resembles sitting in a bucket being whirled around somebody's head. And this has nothing to do with pendulums once the bucket is rotating.


text:
What is the purpose of a retracting cowl?
I flew yesterday in a SWA Boeing 737-300(?) in the seat next to the engine. Right after touchdown, I noticed that part of the cowl retracted back significantly (a few feet or so it seemed), exposing the engine body. The cowl closed back when the speed dropped to taxi levels.
What is this cowl retraction for? Is it part of the aerobraking?

You have seen one type of the possible thrust reversal systems in action. In the following image is the bottom one.

Image source
They deploy surfaces that deflect the bypass air and aid the brakes in slowing down the aircraft after touchdown.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
What are the ICAO PANS? What are their contents?
There are how many I.C.A.O. PANS? What are their basic contents? I have tried a lot in internet, but could not find out.

possible duplicate of Where can the ICAO recommendations be found?

what are their basic contents not a duplicate IMO :)

Not a duplicate. This questions is specifically about the PANS, not the entirety of the ICAO SARPs

ICAO Procedures for Air Navigation Services (PANS) are documents approved by the ICAO counsel and recommended to states for worldwide application. As such they attempt to make air navigation services uniform across the world.
There are six PANS:
Procedures for Air Navigation Services - Air Traffic Management (ICAO document 4444, a.k.a. PANS-ATM)
Specifies the actual procedures to be used by air traffic service units in providing the various air traffic services to air traffic.

Procedures for Air Navigation Services - Aircraft Operations  (ICAO document 8168, a.k.a. PANS-OPS)
Lays down rules for designing instrument approach and departure procedures. Such procedures are used to allow aircraft to land and take off when instrument meteorological conditions (IMC) impose instrument flight rules (IFR).

Procedures for Air Navigation Services - Abbreviations and Codes  (ICAO document 8400, a.k.a. PANS-ABC)
Is a list of abbreviations and codes commonly used in aviation.

Procedures for Air Navigation Services - Aerodromes  (ICAO document 9981, a.k.a. PANS-ADR)
This details aerodrome licensing and Aerodrome Manuals

Procedures for Air Navigation Services - Aeronautical Information Management  (ICAO document 10066, a.k.a. PANS-AIM)
The PANS AIM details the requirements for collection, management and provision of aeronautical data and aeronautical information. It specifies the procedures to be applied by AIM units in providing the various aeronautical information services to other states and aviation stakeholders.

Procedures for Air Navigation Services - Training  (ICAO document 9896, a.k.a. PANS-TRG)
PANS-TRG specifies the actual
procedures to be applied by training organisations when
providing training for aeronautical personnel.
It details ISD (instructional Systems Design and Systems Approach to Training (SAT) which enables criterion based and material dependent methodical learning

Pans are some documents giving information about air navigation provide by council his role to make the application of rules concerning the air uniform across the world

Welcome to aviation.SE! Could you please consider expanding your answer? As it stands now you are not adding much information to the already exiting one.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Does the FDR record data about the cockpit door lock system?
Does the flight data recorder record whether the cockpit door lock was deliberately used to lock someone out?

How could one detect the reason why the door was locked? The best you could do is record the fact that it was locked.

@terdon it is possible to override the door lock from the inside even if the correct keycode is entered on the outside.

@terdon the edits changed the original question somewhat

@Simon Your edit unnecessarily removed significant parts of the question. This is not accident speculation; it does not put forward or ask for an opinion on any particular theory. Without the specification of A320 in Europe, this question is a lot more general. We do allow accident-specific questions on here.

While it is not on the required parameters list, it would not surprise me if it would be recorded- most FDRs record considerably more than the 88 mandatory parameters, and cockpit doors are of course interesting in accident investigations.
That being said, it should also come across pretty evidently on the Cockpit Voice Recorder if somebody was trying to get in. The normal request is a pretty distinct buzzer and the emergency request is the same buzzer for 5 minutes unless denied/ cancelled by the pilot. The door also locks quite loudly. Both should be able to be picked up by the general cockpit area microphone (CAM).

Good point about the sounds.

ICAO Annex 6 part 1 (Appendix 8) lists parameters required to be recorded on digital flight data recorders.  This should give you a general sense of what gets recorded, but cockpit door locking state is not among them.
The caveat to this is many of these might not be individually codified in regulation by member parties to the convention.  Carriers are regulated by their state of registration.  In the United States, cockpit door locking state is not one of the required parameters mandated by the FAA.
(The original answer specifically mentioned European regulations, but has since changed to a general question.  My answer has been edited accordingly.)

And as most of those locks are simple mechanical latches, there being no electrical circuits involved, no sensors and wiring leading to the door, it'd be pretty hard to detect and record, and a major investment would be required to install such a system and get it certified.

@jwenting at least in an A320 seems like the lock is actuated with an electrical switch.

I don't see why you specified European or A320. Neither were in the question.

@raptortech97 The question contained both European and A320 until it was edited by other people to remove that. And I don't see why somebody shouldn't mention Europe in their answer. Not everyone lives in the USA, you know.

@DavidRicherby Thanks, I didn't see that. And I'm not suggesting that everyone should always talk about the US, but the answer specifically gave the answer for the FAA with the disclaimer "Not that this would translate precisely to European carriers," which was kinda out-of-the blue.


text:
Do airlines black-list certain movies on their inflight entertainment system?
Do airlines trust the common sense of the responsible employee, when it comes down to NOT making certain movies available?
Besides excluding movies not appropriate for minors, are movies like United 93, Snakes on a plane or Alive officially a no-no?
Otherwise, without an official directive against these movies, aren't airlines risking a blunder and sparking an Internet storm?

Any such procedure for choosing in-flight movies would be an internal procedure of the airline in question, and not part of their official published policy. Also, any movie shown on a plane would have to be properly licensed to be shown in that way, which means the movie selections go through their legal department, so it's probably just not some random employee clicking tick boxes for movies to show in flight.

Yes, they can show anything except "Airplane".

I've been looking for CDs of those movies for a while. Nice film to watch!!!


text:
Can commercial airliners fly over hurricanes?
When Patricia was making landfall yesterday I opened up Flight Radar 24 to see if there were any planes flying around or near it.  As I had expected, there were a couple of storm watching planes near the hurricane, but what surprised me is that there were some commercial flights that seemed to be going over portions of the hurricane...
So, I'm curious, can commercial aircraft go high enough to simply fly over a hurricane, assuming they take off and land at airports that are not effected by the storm?  Or perhaps they can only safely fly over the fringes and need to stay away from the eye?
Basically, can airliners fly over hurricanes, and if so can they fly over all of it, or just parts?

I was in Cancun when a hurricane was about to hit, and all the commercial outbound flights were full so I chartered a bizjet to take my family out. We took off and flew much closer to the 'cane than I expected, and by the time we were at FL250 it was clear skies and we could look down and see the massive spiral.

^^ a pressurization fail would have been interesting....

Check out https://thepointsguy.com/news/can-planes-fly-through-thunderstorms-and-hurricanes/ for an instructive article on flying over hurricanes.

In general, no- except in emergencies or 'small' hurricanes.

Commercial aircraft usually fly around bad weather (like hurricanes), not over it.
Some of the hurricanes can reach upto 50,000+ ft height, making it all but impossible for most modern commercial aircraft to fly over them (except maybe you're flying a Concorde). Even for smaller storms, the convective process over the hurricane will cause severe turbulence, affecting overflying flights.
Even for hurricanes where the  aircraft can fly over, there is the problem of safety- What will happen is something happens (for example engine failure) and the aircraft has to descend to lower altitudes? Obviously no one wants to descend into the storm.
There are some aircraft that do fly into hurricanes for scientific experiments. These are crewed by trained (and experienced) specialists and involve a lot of detailed planning. For example, the aircraft are equipped with radar that helps in avoiding the worst parts of the storm. Still, it is risky and the aircraft is exposed to significant loads and vibrations not usually exposed in commercial service.

Patricia sure wouldn't fit the definition of "small hurricane." They're saying it's the "strongest storm ever measured on the planet."

This popped to the top of the active list today with Irene about to hit Florida. I was going to question your use of "Patricia" when I realized the date of the comment. They're now saying the same thing about Irene.

@FreeMan You mean Irma. Irene was in 2011.

Um, yeah, Irma... nearly the same... :/

A hurricane can reach up into the tropopause at 50,000+ feet, making it impossible for a modern airliner to overfly it.
Even if you could overfly it, you're in a difficult situation where if anything goes wrong (i.e., requiring a descent), your options are much more limited since hurricanes are so large in area.
There are circumstances where you would fly near the edge of the hurricane in order to take off or land before the main part hits.

Concerning "flying near the edge of a hurricane", check out Delta 431/302 in & out of San Juan during Hurricane Irma in 2017.

Can commercial airliners fly over hurricanes?
They can fly through the top (or even the base) of the hurricane in an emergency situation and nothing will happen, see
Flying through a hurricane (YouTube), but it is not preferred to expose the plane to such vibrations if better options, like going around the hurricane, are available.
The critical point is crossing the edge of the hurricane eye. However, an airplane can pass through that zone of high winds in minutes.

I don't know who voted this down, it's some pretty useful information.  It's not the whole answer, but it covers a particular circumstance I'm curious about.

It may only take a short time to pass through the eye wall, but even with trained crew and lots of planning, things still go wrong.

I don't know that I would say "nothing will happen." The Hurricane Hunters are trained for this. Everyone else will generally avoid it for a lot of reasons, not the least of those being that the storms are often quite tall, well above the service ceiling of most modern airliners.

Flying through serious turbulences (see: TURBULENCE on Flight BA 244, You Tube) is as dangerous as flying through a hurricane as long as both phenomena induce the same level of vibrations in the plane. Not the speed of the wind is the problem for a plane but the gradient of speeds. If a plane comes from region 1 with the windspeed Vw1 and gets to region 2 with Vw2 there will be a sudden variation in lift, like a punch, that will quickly disaper because lift is independent of the wind speed, excepting transitions zones.

@RobertWerner: Isn't flying through an extratropical (cold-core) storm more dangerous than flying through most hurricanes, precisely because of the greater turbulence?


text:
Is it possible for an airliner to safely fly with doors open?
From another question asks about the possibility of dropping bombs from converted airliners. My question: is it possible to safely fly aircraft with a door open for the whole flight envelope? Would the aerodynamics be affected if we did not close the door?
I am not asking about the need of oxygen for the crew, just about the aerodynamics involved. I know the plane would not be pressurized.

vasin, I changed the title to specify "an airliner," since that's what your question body and tags say. In this question, the answers differ quite a bit between those cases (it's completely a non-incident to open the door of the PA-28 I fly in flight, for example, and my instructor did so multiple times.) If you're interested in non-airliners, too, feel free to edit it back or ask another question.

It will add some drag, for sure, but C-130's regularly fly with their back doors open which you see in any movie where material or parachuters are jumping out the back.  https://chivethebrigade.files.wordpress.com/2013/02/c-130-back-door-920-81.jpg

Flying with the rear door open isn't limited to military airlifters -- Skydive Perris has a DC-9-21 that had its rear airstairs removed and uses it for skydiving operations, taking folks up to 13,000' for a real D.B. Cooper experience.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a modified 747 that has an 5.5 m x 4.1 m door that is opened during flight for the installed in infrared telescope. But the door is usually closed during takeoff and landing.
But according to this story there are emergency procedures to land with an open door and it had to do so once, when it became stuck half open.

Images courtesy Wikipedia

Is the door closed at takeoff and landing out of aerodynamic concerns, or just to protect the equipment from bird strikes and foreign objects on the runway?

@JeffreyBosboom: There are other concerns as well for closing the door. For instance, IR telescopes must run very cool, so closing the door allows the team to cool the telescope cooler than ambient before take off, and additionally to flood the "trunk" with nitrogen to prevent condensation when returning to a higher-temperature environment on landing.

This seems to me the only "correct" answer, as it's the only one assuming a modern airliner opening a door in a way that the engineers considered "safe". Having survived structural damage (which other answers point at) doesn't equal "safe operation" the way I understood the question. Nice find!

@DevSolar I'd assume that this section of this aircraft is not within the pressure vessel, though (since the crew probably wants to breathe while it's open,) so it's quite different from opening a door in flight in a normal airliner.

@DevSolar I modified my answer to mention an actual (non-modified) airliner that did fly with a door open without any structural damage involved.

@reirab: You are correct.  Quoth Wikipedia: "The telescope is mounted in the aft end of the fuselage behind a pressurized bulkhead. The telescope's focal point is located at a science instruments suite in the pressurized, center section of the fuselage, requiring part of the telescope to pass through the pressure bulkhead."

Aloha Airlines Flight 243 proved that it's possible for a plane to fly with about 25% of its roof missing. The door, however, appears to have remained closed until landing.
Image courtesy Wikipedia

This does indeed imply that a door missing might also be survivable.

The question specifies fly safely. Aloha 243 was not safe: it resulted in one death and injuries to about two-thirds of the passengers. Having said that, it's unclear how many of the injuries were as an immediate result of the roof coming off and how many were sustained by flying around in a plane with no roof.

Valid point, @DavidRicherby, however if one considers safe to be survivable then this seems to be a reasonable example. (After all, don't pilots consider any landing that you can walk away from is a good one?) Not trying to minimize the death or injuries, just making a point about an extreme example of the OPs question

Virtually no one considers "survivable" to be "safe".

@FreeMan Of course I don't consider "safe" to mean "survivable"! That would be a ludicrous definition of "safe". If somebody says to you, "Have a safe flight!" do you think they mean "Have a flight where the roof blows off the aircraft, one of the flight attendants falls 24,000ft into the ocean, eight other people are seriously injured, almost everyone else receives minor injuries and the plane is written off?" I don't. I guess I expect more of an airline than you do.

@DavidRicherby the aircraft flew safely. The fatality happened due to the roof breaking open during flight in an uncontrolled manner, not due to there being no roof.

Answer does not answer the question, door is intact during the flight.

@jwenting: The aircraft flew, full stop. I very much doubt that there was anything "safe" about it, structurally or aerodynamically. (They wouldn't take off in it again, now would they?) I don't think that "not crashed" equals "safe".

@DavidRicherby Sorry but you reminded me of this... http://youtu.be/WcU4t6zRAKg

@DevSolar safely as in "no further fatalities and injuries" and it didn't crash. Of course the incident left it structurally unsound, but that wasn't so much the result of flying with a hole in the roof as much as the roof tearing away in the first place.

@jwenting: It was structurally unsound to begin with, due to extensive fatigue damage.  As were three of Aloha's other 737s, two of which were scrapped and one repaired.

@Sean of course, but that soundness got even worse with the roof gone, as parts of the aircraft that had not been damaged by fatigue got exposed to severe stresses.

@jwenting: ...there were parts of that aircraft's fuselage that hadn't already been severely damaged by fatigue?

Check out my answer here to a similar question (about what would happen if you opened a door in flight). In short it depends largely on the plane but in the most general way of looking at it. You could do it up until you would need to pressurize the plane (or you would have to wear $\rm O_2$ masks or something to be able to breathe).
UA Flight 811 was able to make a safe descent and return to the airport after its door blew out, so large planes are maneuverable without a door.

Planes like the DC-3 used in WWII for paratrooping regularly ran with their doors open.

If you consider rear cargo ramps doors, the C-130 can open its rear ramp for HALO jumping at high altitudes.

In typical HALO/HAHO insertions the troops jump from altitudes between 15,000 feet (4,600 m) and 35,000 feet (11,000 m)

There is at least one other powered glider I can think of that can open its doors mid-flight...

The DC-3 example should pretty much answer this question in the affirmative.

Agreed however there are other cases involving pressurized planes mentioned here that should not be over looked.

Your last picture (Spaceshuttle) may very well be taken outside the atmosphere. In my opinion, it has nothing to do with the question unless you provide an example of a Spaceshuttle opening it's doors during atmospheric flight.

@Mast - I think that last picture was meant with tongue firmly in cheek. I thought it was funny, at least.

@Mast, Floris is correct it was meant to add some humor, the rest of my answer is on topic and through enough.

The answer is definitely on-topic afar from that. Especially the C130 is a good thought. It's easy to miss paratrooper planes often have their doors open for longer periods of time.

Well, the question was actually tagged "airliner" even before I edited that into the title, so I don't think the C-130 really counts there, though it is a good point nonetheless. The question would be very broad if it included any airplane, since the consequences of opening the door on a pressurized or an unpressurized aircraft are quite different. Also, I'm not quite sure that the last picture there really counts as 'flight.' ;-)

There is a whole section of the FAR that covers space flight and it is very much considered flight. You actually need up through your instrument rating to fly the shuttle. http://www.ecfr.gov/cgi-bin/text-idx?SID=2abf0c470116714c2fe8c9e875ece5c0&mc=true&node=pt14.4.460&rgn=div5#se14.4.460_15

@Dave, interesting that it doesn't seem to require a multi-engine rating.

@Dave It looks like the FARs are more concerned with the atmospheric portion of the operations (launch and reentry,) which much more legitimately count as flight. Operations actually in space seem much more questionable to consider 'flight,' given the lack of an atmosphere in which to fly.

@FreeMan Well, the FARs are for spaceflight in general, not just the shuttle. There are single-engine rockets. :)

Since comments mentioned that not all of the outcomes from Aloha 243 were exactly safe, here's another example that actually was quite safe:

Southwest 812 had a large hole open in the roof during flight at cruise altitude, resulting in rapid depressurization at 34,000 ft. 2 of the 123 people on board (a flight attendant and a passenger) suffered minor injuries. The plane diverted to Yuma, Arizona and landed safely about 26 minutes after the depressurization. As with the other cases, the doors were technically shut, but I'm not so sure that that made much difference.
Furthermore, this was not the first time this had happened. Less than 2 years before this incident, Southwest 2294 had a similar incident (though with a smaller hole) and safely diverted to Charleston, WV with no injuries at all.
What's more: according to the FAA's registry and airfleets.net, it looks like both of these aircraft were returned to service and are still actively flying for Southwest! So, it looks like both the "you can walk away from it" requirement and the "the aircraft is reusable" preference were met in both instances.

The Boeing 727
Another case that actually doesn't involve any structural damage at all is the 727 and its airstair. In the famous case of D.B. Cooper's hijacking, this occurred while the aircraft was pressurized. According to the wiki on the incident, the result was the following:

The crew soon noticed a subjective change of air pressure, indicating that the aft door was open.
At approximately 8:13 pm the aircraft's tail section sustained a sudden upward movement, significant enough to require trimming to bring the plane back to level flight.[33] At approximately 10:15 pm Scott and Rataczak landed the 727, with the aft airstair still deployed, at Reno Airport.

Additionally, some skydiving clubs even offered dives from the 727 for many years, though these presumably didn't pressurize the cabin. I'm not sure whether this is still offered.

Well done! I just went with the one I knew of.

Yes they can, here's a Convair 880 with cargo door open.

DC-8 with cargo door open.

Was this from a flight-test, perhaps?

According to this it was at MIA in 1979. http://www.airliners.net/aviation-forums/general_aviation/read.main/866590/ and the DC-8 is also at MIA in 1989  http://www.airliners.net/photo/203157/L/

@UnrecognizedFallingObject they were the result of the door not being properly latched and coming open during flight.

@jwenting -- oops!

@SportsRacer: Must be the salt air corroding the latches...

For the whole flight envelope, maybe not. Depends on the door design. Gear doors frequently have speed limits before they fly off on their own. Large openings create substantial drag, so we may find that the envelope is self-limiting: the aircraft can no longer reach speeds and/or altitudes where the opening is a big problem.
There are too many variables for a simple answer.

gear doors have speed limits because they extend into the airflow during flight. The cabin doors would rotate inward, keeping them out of the high speed air flowing past the outside. It would probably get rather noisy... A greater concern I have is maintaining structural integrity for long periods on an aircraft that has a cabin designed to be used while pressurised. This would mean the aircraft can only operate at low pressure altitude with the door open.

@jwenting Why would being unpressurized affect the structural integrity of the aircraft? Wouldn't the aircraft actually experience much less stress in the case of being unpressurized (until it crashed due to unconscious pilots, at least?) There have been quite a few instances of pressurized aircraft becoming depressurized and continuing at cruise flight levels (due to incapacitated pilots) and I don't recall any structural problems in any of them, prior to the aircraft running out of fuel and flying into terrain. The occupants may experience lots of stress, but the airframe shouldn't.

@reirab It's not the lack of pressurisation I'm concerned with, it's the dynamic stresses on things like the door frames from the turbulent airflow that would result from the doors being open during flight. They're not designed to handle those stresses in most aircraft.

@jwenting: Or, potentially, the dynamic stresses from the process of depressurisation.

Biplanes and ultra-lights don't have any doors at all. In general, for any plane the wing, the empennage and, to a lesser extent, the surface area on the lower part of the fuselage is all that is important. If you installed structure-reinforcing crossbars you could remove the entire fuselage wall of a commercial jet and it would still fly.

[Citation needed]. A reinforced airliner with the fuselage wall removed would have a lot of drag. And how much of the fuselage wall is needed for strength? (Admittedly, I assume there'll be no cargo or passengers so you could afford to weaken the structure somewhat.)

@DavidRicherby Note that I specifically said "structure-reinforcing crossbars" would be needed. He just asked if it would fly, not if it would fly well.

Also note that the question specified "airliner". Not too many ultra-light airliners that I'm aware of, though I may be wrong.

3 seats is the most I can find in anything that qualifies as an ultralight.

authors: Валерий Свердловский


text:
Can pilots hear a car horn while on the ground?
In this question I asked about the pilot's ability to give a signal to others nearby while on the ground. My question here is, if someone else wants to get the pilot's attention on the ground, could the pilot hear someone honking a car horn from the cockpit?
Edit: thanks for all comments. I want to ask about a situation involved large aircraft, engine running, car is near the aircraft. Such as in large airport operation. My question comes from series Ultimate Airport: Dubai when a trainee is driven to different areas of airport as part of his certification. I wonder if he can call attention of pilot in cockpit by honking the horn. I am aware that more standard mode of communication such as radio, flare or ground marshall is preferable.

What kind of aircraft and where is it that it needs to hear it? Are the engines running? Where is the car in relation to the aircraft?

Large aircraft, engine running, car is near the aircraft. Such as in large airport operation.

I think that's a firm maybe.

Pilots and airport vehicles will probably communicate by radio using the ground frequency. Remember that vehicles are moving under the control of the ground control/tower in large airports.

To continue @RonBeyer's comments:  is the engine electric?  And what kind of horn?  A regular horn is harder to hear than when someone puts a "train horn" on there car.  There are... a lot of variables not being accounted for here.

Flares can be used to contact a pilot in an emergency, ie a red flare to imply take off forbidden.

Maybe a glider or a hang glider?

Airplane engines are loud, so if they are running I would say no, a pilot is not going to hear a car horn. If you want to get a pilot's attention without a radio you'd use light or hand signals.


text:
How should I request an IFR approach at the end of a VFR flight?
Here's the scenario:
The flight starts night VFR, with broken ceiling at destination (class C airspace) and expected to improve according to the pre-flight abbreviated briefing. I'm IFR certified but prefer to stay VFR to dodge icy clouds along the way.
Now I'm about 15nm from my destination, talking to approach control, and the ATIS calls the ceiling overcast: it's apparent I'll have to file IFR one way or another. Approach is busy, but not overwhelmed.
What is the best way to handle this situation on the radio? I have the ATIS, picked an approach and have a squawk code (advisories):
a). Do I ask approach directly for the IFR clearance, and what is the officially sanctioned phraseology? Also: do I have to cancel IFR when I'm on the ground/see the runway, i.e., is the clearance to descend thru the layer and shoot an approach a 'real' IFR flight plan?
b). Ask approach if I can change frequency to FSS, file with them, then return to approach to pickup the clearance (and cancel when on the ground/view of runway)?
c). Other?
I did read How do you request a "pop up" IFR clearance?. In my scenario I have the time to call FSS, there is no emergency, I'm on flight following and could complete (b) if that's the best way to go.

You will also be told to report IFR cancellation in the air of on the ground with approach, otherwise with the one-in one-out rule, nobody else can land at the same field until you cancel your IFR.

It's called "popup IFR" but there's no guarantee that ATC will grant it base on their workload, afternoon IFR flight plans are supposed to be submitted an hour in advance. If you expect IFR at your destination then file an IFR flight plan. The flight plan doesn't have to start at an airport and can start at an intermediate fix or navaid near your destination.

Since you are already on flight following you are already in the system (ATC has a flight strip for you and a datablock on the radar).  Just call ATC and ask them for IFR to your destination.  Don't bother asking for a frequency change to FSS to file a flight plan, only to come right back on and ask to pick it up.

Austin Approach, Cessna 12345, request IFR to Austin

Since you have flight following ATC already knows your type, equipment suffix, location and altitude, so don't repeat it.  ATC should ask

Cessna 12345, verify you are qualified for and capable of IFR flight?1

and after you (hopefully) reply in the affirmative will issue:

Cessna 12345, cleared to the Austin Bergstrom Airport via fly heading 270, vector to ILS Runway 35-Right final approach course, maintain 2000.

If you are already in the approach environment you probably won't get a climb to an IFR altitude, you'll just be given whatever altitude they need for the arrival flow.
This method is faster than filing with FSS and easier for ATC (they don't have to approve a frequency change, wait around for you to file, then have you come back and wait further while your flight plan works its way through the system).  However, bear in mind that none of the "extra" search-and-rescue information you would give to FSS (fuel and souls on board, emergency equipment, filed alternate, aircraft color and markings) will be recorded.2
If you are landing at a towered airport you will not need to cancel IFR as the local controller will do this for you.  If you are landing at a non-towered airport you will need to radio ATC or FSS or telephone FSS to cancel your IFR service.

Footnotes
1This is a relatively new requirement after a non-IFR-rated pilot accepted an IFR clearance and subsequently crashed.
2Some facilities will have you rattle all that information off on the controller's frequency or a separate unused frequency and then mark that down in the flight plan remarks, just to have it on the tapes in case it's needed. If you're only 15 miles from your destination they probably won't do this but it's a possibility.

I would add to this that (if possible & necessary) wait until you have been switched from center to approach before asking as the process on their end is different. If you are on Center and ask for the approach, they are required (for loss of comm procedures) to clear you for the entire approach including getting you to the IAF whereas approach can clear you to the IF. I do this all the time flying from Phoenix to San Diego where the overcast has come in and I need to shoot the ILS to KCRQ. If I request IFR from LA Center I get JLI-OCN-HOMILY-ESCON but from SoCal I get direct to ESCON.

Note that as of 2020, you call ATC directly (not FSS) to cancel IFR after landing at a non-towered airport. The phone number should be in the A/FD remarks for any airport without an existing RCO/CD freq.

See also this question about popup IFR
"Back in the day" (began the grey bearded geezer who hasn't flown IFR in a long time), you'd just tell Approach something like
"Approach, Cessna 123, (possibly insert location info),  request IFR, approach XYZ".
The rule I was taught is "who are you, where are you, what do you want".
You already have a squawk & you're already talking, so it should be simple.  I'd expect to hear back "Cessna 123, IFR approved, maintain current heading, ..."
If I recall correctly, Tower or Ground closes your IFR plan for you - they know you landed.

I missed the bit about being 15 nm out.  My concern is how long would it take FSS to notify Approach that you want IFR.  Since you're already talking to Approach directly, my inclination would be to cut out the middle man.  I may be wrong :-)

Just saw your response. That's what I would do too, less work for all, but there seems to be a stigma -if that's the right word- about the pop-up so close to the destination. Maybe I should file further out when I get the first inkling that the ceiling isn't improving (can't upvote yet)

...ATC still has to fit me in, I understand that...

Controller workflow and congestion are a consideration. If the work load is light, short notice may work fine.  If the workload is high, then I usually ask if it would be possible to work me in.  One time, I was given a hold with a 45 minute EFC time, but the weather went south unexpectedly and there was allot of traffic. There was also a plane with a radio problem on frequency. As it turned out, I only held for and entry and one loop, once the controller got things worked out.

If you're in the Center environment (realizing that you specifically said approach control), you can expect to be granted an IFR clearance based on workload. A Center controller would have the option of telling you to remain VFR, and if that means turning around, that's what you do.
Realistically, however, there's a very good chance you'll get your IFR.
The controller may ask if you are capable and qualified for IFR flight, but this is usually asked in distress situations. If you call me and ASK for IFR, you've just informed me that you are capable (aircraft is outfitted for IFR) and qualified (you're an actual IFR pilot), just as if you had just taken off VFR and requested right after you got the wheels up.
There are other caveats.
Filing through radio while in the air BEFORE you get close in will normally get you quicker handling.
In Denver Center, terrain is always an issue, and VFR aircraft often fly below our minima. Be prepared for the need to climb in VFR conditions to our minimum vectoring altitude, which is higher than what is prescribed in 14 CFR 91.177.
Also, you'll normally get lower priority than IFR aircraft going to the same destination.


text:
Does anybody recognize these wings?
I was on a tour at the University of Maryland, and I saw these wings sitting against the wall:

I asked the guide, and he had no idea. (In case you can't tell these are two wings. The green lines indicate the extent of the two wings.)
What aircraft do these belong to?

For those who are voting to close as "Unclear". This is as clear as it can get.

@ptgflyer: Is that photo taken at the College Park Aviation Museum?

@Mike Sowsun: No, it's in one of the labs.

There is a wingless Ercoupe on display at the College Park Aviation Museum. I wonder if those wings belong to it.   http://pierregillard.zenfolio.com/img/s11/v32/p1391446624-2.jpg

You would have to talk to UMD. I was just touring them.

Those wings are from an ERCO Ercoupe. The diagonal ribs are very distinctive.

Interesting: the first photo you supplied shows a wooden wing structure, whereas the remaining photos—as with the photo in the question—show metal structure; they all appear to have the same shape. Looks like the aircraft was built with wood during WWII due to aluminum shortages. Or perhaps it wasn't built during the war, but only served as a proof of concept?

Yes, they used wood in WWII.

This required super-power in identification. Standing ovation!


text:
What is the range of TCAS?
The traffic collision avoidance system has two versions, TCAS I and TCAS II.
I know for sure that TCAS I has a range of 40 nautical miles, but what is the range for TCAS II?

Short answer
If a single range should be stated for ACAS II, it would be the reliable range of 14 NM, even if targets can displayed by current TCAS II up to 30/40 NM.
Reliable means here: a track can be established with a probability of at least 90 per cent for aircraft within the surveillance range. The reliable range of 14 NM is achievable with a ground plane antenna, a power of 250 W and a Mode S sensitivity of –74 dBm.
The mean number of aircraft in a spherical volume is proportional to the cubed radius. Each aircraft tracked by a ACAS system must be interrogated, and its position/altitude data stored. This creates stress both on the ACAS equipment and on the radio spectrum used for ATC (SSR and ADS-B in addition of ACAS). The current balance between stress and benefit is a range of 14NM which can be reduced to 4.5 NM if the number of aircraft or the number of ground interrogators prevent managing the nominal range. The spectrum stress should be relieved with the forthcoming ACAS-X.
In addition, while the nominal monitoring range is 14 NM and the guaranteed range is 4.5 NM, RA/TA are issued based on estimated time before separation failure (more accurately before the closest point of approach):

From Eurocontrol ACAS guide
(In the answer, ACAS is related the principle, TCAS is the technical implementation of the principle.)

Diversity of ranges
There are different range values that can be considered:

Overall pulse detection range is 30 NM.
Reliable surveillance range is 14 NM.
Recommended RA range is 12 NM.
Guaranteed range is 4.5 NM.

The range must be limited because it impacts the number of transponder addresses to be stored for a given transponder density as well as the number of interrogations within a volume, hence radio spectrum use and reliability.
ACAS II recommendations by ICAO
ACAS II ICAO recommendations are found in Doc 9863, Airborne Collision Avoidance System (ACAS) Manual:

Interrogation pulse detection range is 14 NM for Mode A/C and 30 NM for Mode S.
The equipment should have the capacity for surveillance of any mix of Mode A/C or Mode S targets  up  to  at  least  30  aircraft.
Surveillance must be reliable within 14 NM, with an aircraft density of 0.3 aircraft per square NM within 5 NM (and up to 142 aircraft within 30 NM).
The equipment should assess as possible collision threats only those targets within a maximum range of 12 NM. No target outside this range should be eligible to generate an RA.
As density increases, reliability decreases.
If the overall target count ever exceeds the surveillance capacity at any range up to 14 NM, the long-range targets may be dropped.
The system reliability is guaranteed within the minimum range of 4.5 NM. This radius is deemed adequate for providing protection for up to 500 kt encounter.

TCAS version 7.1
TCAS II is the sole implementation of ACAS II which fully comply with ACAS II. The reference (paying) documents are: DO-185B (RTCA) and ED-143 (EUROCAE).
From the EASA document ACAS Guide

TCAS II can simultaneously track up to 30 aircraft, within a nominal
  range of 14 NM for Mode A/C targets and 30 NM for Mode S targets. In
  implementations that allow for the use of the Mode S extended
  squitter, the normal surveillance range may be increased beyond the
  nominal 14 NM. However, this information is not used for collision
  avoidance purposes.

This is confirmed in FAA document Introduction to TCAS II V7.1

TCAS has a requirement to provide reliable surveillance out to a range
  of 14 nmi and in traffic densities of up to 0.3 aircraft per square
  nautical mile. [...] TCAS can simultaneously track up to 30
  transponder-equipped aircraft within a nominal range of 30 nmi.

The main point is that TCAS range is highly dependent on traffic density and transponder mix (Mode A/C vs. Mode S). I can say based on my experience testing TCAS units in the lab and analyzing flight test data, tracking 30 aircraft within 30 NM for is very reliable.  With a density such that there are 30 or less targets, tracking Mode S is reliable out to the 30 - 40 NM range.  Mode A/C can be tracked out to 15-20 NM in these conditions.

The ICAO numbers are off.  142 a/c within 30 NM is a density of 0.01 a/c per sq NM.  It's closer to the FAA numbers where 0.3 a/c per sq NM equates to 185 a/c within 14 NM  or 136 a/c within 12 NM.

That is true, but the statement in your answer "But this is valid for an aircraft density of 0.3 aircraft per square NM (142 aircraft within 30 NM)." is misleading.  What your latest reference describes is 24 a/c within 5 NM (which is 0.3 a/c per sq NM) and 142 within 30 NM (which is 0.05 a/c per sq NM).  So they are describing a non-uniform distribution modeled around a major airport.  The original statement is poorly made in that it mixes two separate measurements.


text:
What is the regulation related to weather radar use?
I think most commercial aircraft have a weather radar located in the nose cone, like this:

(Source: PhotoBucket, by intelmani5)
It's well known that radar emissions can be harmful, e.g. one could get their eye burnt like an egg in a microwave oven.
However I don't know:

About the typical frequency, power and duty cycle (time in transmission mode vs time in reception mode).
Whether there is a risk for a harmful radiations exposure.

My question is: Based on current radar specifications and regulation, is there an overall ICAO (or local, e.g. US or EU) regulation forbidding the use of the radar on the ground to prevent radiation exposure for people close to the aircraft nose (or conversely to be ON at certain phases of the flight for cumulonimbus detection)?

Too many questions in one topic. It's also very hard to answer as the regulations vary considerably by country. The peak power and duty cycle are dependant upon radar type, so this question can also not be answered.

@sp1: How many questions are allowed if they are related?

You would be better off dividing into several questions - this one is far too broad to give a comprehensive answer.

@sp1: Thanks. I have reduced the number of questions to one.

Aircraft weather radars typically operate in the X-band, which is 8-12.5GHz. Some older radars use C band instead (4-8Ghz).
Within the X-band most operate somewhere around 9.3GHz. For instance, Honeywell's Primus 660 is 9.375GHz while the Rockwell Collins WXR-2100 operates at 9.33GHz.
Regarding the power, that varies a lot depending upon the specific radar and mode of operation. The Primus 660 has a typical output of around 10kW.
To my knowledge there's no specific regulation that completely forbids radar use on the ground but many operators will have their own regulations to protect crews. The radar manufacturers often have similar. To use the Primus 660 example again, page 105 of this gives some recommended safe distances based upon that particular radar and the FAA regulation (which ymb1 linked to in their answer).
-edit-
I just realised I forgot to answer about the duty cycle. The WXR-2100 gives some fairly typical numbers for this: Pulses between 1-25 microseconds and a repetition rate of up to 3000/sec. In the absolute maximum case that means that the radar will be transmitting 7.5% of the time and in normal operation it'll be much less than that (in my experience more like 1-2%).

Thx, now fixed.


text:
Why did this aircraft re-route?
I was looking through my ADSB Virtual Radar outputs couple days ago and saw a weird re-route for one of the aircraft that looked out of place.
Can one of the pilots/ATC guys pitch in to help me understand what may have caused this A/C to do this?  Is this "normal"?
I would expect if this was a wake turbulence from A/C ahead they would have just asked the rerouting A/C to just slow down?
Tail #: N39463

See this question: Why are vectors used for sequencing rather than speed control?.

Its hard to tell from the picture, but assuming you are talking about the yellow airplane and the "wiggle" in the course, that was probably just an S-turn for spacing.  Sometimes the controllers don't get the spacing exactly right, or one airplane slows down early and messes things up.  In those cases, in visual conditions,  the controller can ask the following airplane to make an S-turn.  This causes the plane to take a bit more time to cover the same distance as flying straight and spaces the planes out.
This may be used instead of a speed assignment for a number of reasons

Planes on approach may not be able to slow down further as configured
There may already be a speed assignment but it wasn't enough
Slowing down takes time, and S turn may be more efficient.

An ATC instruction may assign a heading and follow up with a new heading and approach clearance or other instructions to join final, or they may simply ask for an "S turn to the right for spacing".  The vectors would be more likely in instrument conditions and the S-turn more likely in visual conditions.

So you know I am going to ask, why not just have the A/C slow down?
Also, what might that ATC instruction sound like?

+1. This sort of maneuver is much more efficient (and doesn't require altitude changes) than changing the speed of the aircraft would require.

@user1599043 updated.

@casey - learned something new, thank you!  I know this is probably really basic, but neat to hear some of the explanations for things i see on my scope :)

@user1599043 thats what we're here for!


text:
Routers on airplanes
I think this is an okay place to ask this. Let me know if it is not.
In about a month, my school is going on a school trip and the plane ride is going to be around 15 hours. I was think about setting up a small network on the plane so that we can chat on a locally hosted program.
I would do this by bringing a small router and hooking it up.
Is this against the rules of most airlines and do you think I could get in trouble?

I'm guessing you are considering a wireless router, because even if you are sitting together, Ethernet cords everywhere have their own issues.

You need to check with the airline. Some airplanes (especially international) have in-inflight wifi, so you should be fine using that. If it doesn't have in-flight wifi, it might not be rated for wifi, in which case the answer is probably no.

@raptortech97 I'm hoping that if they do have in-flight WiFi, it doesn't allow you to directly connect with other devices on that network.

@fooot yeah I don't know how router security works at all. But isn't there some way it could work? With like..  a VPN I think??

@raptortech97 software developer here, it is possible (and probable) that the in-flight WiFi does not allow client-to-client connection. In that case you'll need to turn the WiFi connector of your laptop into a router, which isn't that hard really (lookup "netsh wlan start hostednetwork").

@kevin so does that mean that bringing a wireless router is not breaking the regulations?

I dunno about the rules, but a WiFi router is technically the same as the WiFi adapter on any laptop : a transmitter + receiver. You just need the software to make it function one way or the other. Laptop is more convenient as you don't need A/C power.


text:
What is an FAA Notice? Does it have legal force?
The FAA issues Notices and other non-legislative rules (e.g. Advisory Circulars). These documents are distinct from the laws in Title 14 of the Code of Federal Regulations. What are the differences between them? Do they have legal force?

I don't know if it helps, but this page separates all FAA documents into categories

Good find. I'm hoping there's a similar page that includes definitions and details of the usage of each type of document.

As SSumner pointed out, the FAA produces all kinds of paper - some of which is "regulatory" and/or have legal force, and some of which isn't.  I won't go into all of the specific document categories, but I'll talk about a few that most of us will find interesting.

Handbooks and Manuals
The FAA actually publishes full-length books, if you know where to look!
The FAA has written books on flying an airplane,  flying balloons, risk management, buying & owning an airplane, and a whole bunch of other topics which provide broad information that you can use to familiarize yourself with a topic.
Advisory Circulars
Advisory Circulars (ACs) are the FAA's way of explaining their regulations.
They're broken down into sections which generally match up with the FARs they're interpreting.  Some ACs may rival the lengths of handbooks, but they are much more narrowly focused, generally dealing with a single topic and often much easier to understand than the legislation they're interpreting.
Airworthiness Directives
When an unsafe condition is identified in an aircraft or component which requires action the FAA can issue an Airworthiness Directive (AD), requiring that specific actions be taken.
Often the required action is compliance with a manufacturer's Service Bulletin (or compliance with a service bulletin is also an acceptable method of compliance with an AD).
There are also Special Airworthiness Information Bulletins (SAIBs), which are like "Mini-ADs": The FAA is calling operators' and mechanics' attention to something which may not merit an AD. SAIBs are informational - you don't have to comply with them - but it's certainly a good idea to read them and pay attention to the things they're highlighting.
Orders
An "Order" describes procedures used by the FAA. For example, JO 7110.65U describes various aspects of how ATC works.
Procedures described in Orders may be modified by Notices.
Notices
A "Notice" is just an agency's way of saying "Yo! Pay attention! Something's happening!"
Your question seems generally concerned with internal notices which I linked to in the title. These generally modify Orders, like this one.
The FAA can also issue "Public Notices" about changes that affect airmen and the   general public, or pretty much any other topic they'd like.  Public Notices are published on faa.gov as well, but the easiest place to find those is through the FAA Safety Team's SPANS system which is searchable and has a much nicer interface than digging through the FAA site by region.

As for whether the non-legislative documents have legal force the answer is a very well-qualified "maybe".  Some certainly have the force of regulation – Airworthiness Directives being the most obvious – and for our purposes I think that's what you're really concerned with, because the law says you have to comply with the regulations.
Orders and Notices are similarly "regulatory" for the agency: The FAA has to comply with its own orders, so ATC has to be operated the way JO 7110 says it's to be operated.
For stuff like ACs, Handbooks, etc. we discussed this in chat a while back talking about the answer to FAA Order 8900 and the FSMS - the gist of it is that the non-legislative documents the FAA produces are generally interpretations and guidance, but they are "effectively regulatory" in that if you go against them you need to have an excellent reason for doing so, and be prepared to back it up with appropriate justification and evidence if asked.
As a classic example, alterations to aircraft are expected to be performed in accordance with manufacturer's maintenance procedures, or "other methods, techniques, or practices acceptable to the Administrator" which are described in AC 43.13 (43.13-2B deals with light, non-pressurized aircraft and is an interesting read).
A mechanic doesn't have to comply with the guidance in AC 43.13, but if a mechanic were to modify an aircraft without conforming to that guidance they would need to supply supporting documentation as to why their work is acceptable, such as a statement from a Designated Engineering Representative, otherwise the FAA could deem the work "unacceptable" (and from a liability standpoint, the mechanic could wind up in serious trouble should something go wrong with the aircraft as a result of that work).

This is a very clear summary. Thank you.
Could you add links to the FAA documents these definitions are drawn from? For example, this question about Airworthiness Directives cites the FAA definition of that document type.

@JeffStreet As far as I'm aware there's no official FAA "what this is" blurb for any of these other than ADs (if you click on the header link for each category it takes you to the top-level page - ADs have the blurb referenced in that answer, and Handbooks have a few empty words that I didn't find particularly descriptive). The interpretive text above is all me -- which of course means I could be completely wrong and talking out my behind. I'm reasonably sure I captured their meaning & intent with the above, but I'm certainly open to corrections if I screwed up :-)


text:
Can one pilot lock the other out of the cockpit?
One seemingly plausible theory about the MH370 disappearance is that one or other of the pilots locked the other one out of the cockpit and then depressurised the cabin.
However, this answer mentions a code that can be entered to unlock the cockpit door. Could this apply in this case (on a 777)?

Yes, see the Ethiopian Airlines hijacking in February 2014.
From this BBC article about MH370:

The security of the cockpit door offers protection against intruders, but it also prevents action being taken if something does go wrong. Last month the co-pilot of an Ethiopian Airlines flight waited for the pilot to go to the toilet before hijacking the aircraft and flying it to Switzerland.

As for higher authority codes, from the same answer you linked to:

Even if the stronger code is entered that will open the door the pilots still have some time to cancel the unlock request.

Meaning that if a pilot is willfully locking the other out of the cockpit, there is no way for the other to regain access to the cockpit.

They must balance the need for the pilot "outside" to get back in to assist (or overpower) the other pilot, and the need to protect against someone taking the key or otherwise forcing the "outside" pilot to unlock the door for them. It's not an easy choice for the designer to make, and so far they seem to be more concerned with the latter case. If there are more cases of rogue pilots, the choice may swing back to making it easier to enter from the outside. As soon as there are a number of hijackings, the pendulum will again swing...

To clarify: Keeping the door locked if the code is entered requires action by the crew, correct? That is, if the code is entered and the flight crew does nothing, after a delay the door will open? @user7811 Unless you're worried about someone learning or watching the code, and having someone who shouldn't have cockpit access enter the code. You can't magically tell "should this person have access," so you have to decide whether you're more worried about someone breaking into the cockpit or more worried that the person inside will be actively doing something bad.

@cpast, context of user7811's comment (I suspect): http://www.bbc.com/news/world-europe-32062278

@user7811: I'm not incredulous that a pilot in the cockpit can prevent a hijacker with a knife forcing the other pilot, returning from a break, to open the cockpit door for them. Phil Perry's comment seems appropriate to me. Difficult dilemma.

As we all know since this incident, the answer is a definite yes.
It was established later by investigators that Lubitz heavily researched his actions beforehand. I have an eerie feeling he might've looked at this very page.


text:
Why is fuel usage discussed per time, not per distance?
I don't understand why sometimes the fuel used for a flight is discussed in terms of fuel used per time.
I think I have seen it a couple of times here - not sure whether it was in some common context.
I would think that it's much more relevant how much fuel is used per distance in almost every respect, except designing engines.
As an example, this answer to the recent question How do you compare private jet and commercial jet efficiency? uses "x/hr USD" as in "Hourly fuel costs for a private jet can run from 800-2000/hr USD." I'm sure I noticed similar cases a couple of times.
It's fuel cost per time, instead of fuel weight or volume, but that does not make a difference in regards to the question.
Maybe there is a reason based on some "tradition"?

Do you mean for example 10 gal per min?  Is that what you mean by fuel per time?

Yes, or more often tons per hour I think.

Generally, with the same weight, altitude, and indicated airspeed, fuel per unit time should be pretty consistent. With fuel per unit distance, you have to factor in the wind, which generally isn't relevant.

@VolkerSiegel Can you add some examples of where you saw that?

@Farhan I added an example - I have seen it before, in a similar way, but did not take note of the places.

@VolkerSiegel yes, I know.  Just giving an example.

Since I wrote that answer I can tell you why I chose money/time.  it was to make answering the question easier/more relevant since the OP asked question about costs.   But, with that said I always do all my calculations in units of fuel/time (ex. pounds per hour-PPH), since that is what you need to calculate range (winds act for a time) and things like IFR reserves are based on time.

Unlike with driving, in aviation the major variable when planning fuel consumption isn't how far you're traveling but how long you're traveling: Distance traveled in aircraft is a derived value (calculated from groundspeed and time - groundspeed also being a derived value affected by many factors including wind).
A pilot may not readily know their groundspeed (it can be quite different from airspeed), but at a given power setting they know an engine will burn a specific quantity of fuel per unit time, therefore fuel consumption is generally expressed that way (in gallons per hour or pounds per hour), and if they know how long they've been flying they can reliably estimate how much fuel they've burned, how much is left in the tanks, and how much longer they can keep flying before running out of fuel.

To illustrate why this is superior for flight planning consider the following (extremely simplified) flights fitting the following profile:

A 200 nautical mile trip
An airspeed of 100 knots
A fuel burn of 10 gallons per hour
A 25-gallon fuel tank.

Scenario 1: No Wind
With no wind the flight will take two hours to complete.
The groundspeed is 100 knots.
The aircraft will burn 20 gallons of fuel.
The fuel efficiency is 10 miles per gallon.

In this scenario we land with 5 gallons of fuel left in the tank - that corresponds to 30 minutes, which is the minimum legal reserve for day VFR flight per FAA regulations.

Scenario 2: 25 Knot Headwind
With a 25 knot headwind the flight will take about 2:40 minutes to complete.
The groundspeed is 75 knots.
The aircraft will burn 27 gallons of fuel.
The fuel efficiency is 7.5 miles per gallon.

In this scenario we run out of fuel 2:30 into the flight. Running out of fuel is generally considered to be a Bad Thing: The passengers complain, and the FAA asks a lot of uncomfortable questions.

Scenario 3: 25 Knot Tailwind
With a 25 knot tailwind the flight will take about 1:40 minutes to complete.
The groundspeed is 125 knots.
The aircraft will burn 16 gallons of fuel.
The fuel efficiency is 12.5 miles per gallon.

In this scenario we land with almost an hour of fuel still in the tank - a very comfortable reserve.

On longer flights, and in slower aircraft, the effects of wind can be extremely pronounced, and a 5 or 10 knot difference from the forecast winds can mean the difference between arriving at your destination or having to add a fuel stop.

In your edits to the question you noted that operating costs are often expressed as dollars-per-hour as well - this is done for similar reasons: Fuel is a dominating factor in operating costs (and for the reasons above we track that per unit time). In addition many maintenance items are dictated on a time(hours)-in-service basis (oil changes, hot section replacement in turbine engines, inspection cycles, etc.), so it's convenient to break down operating costs that way.

You might want to clarify that the legal reserve under (day) VFR is 30 minutes, not 5 gallons :-)

I would think ground distance traveled would often be a known rather than derived quantity (after all, in many cases the purpose of a flight is to travel between distances a known distance apart), but air distance traveled would be a derived quantity.  In your example, the plane travels 10 nautical miles per gallon through the air, but depending upon tailwind/headwind would have to travel between 160 and 266 nautical miles through the air to move 200 miles on the ground.  I would expect that the airspeed which would yield the optimal number of ground miles per gallon would...

...vary depending upon windspeed, though I'm not sure how much.  Would that expectation be correct in practice?

@supercat The as-flown distance is 200 miles in all of the examples (the aircraft is traveling along a straight (actually great-circle) 200 mile line from point A to point B) - I've included an image with an example flight fitting the hypothetical profile. The way I've always seen flight planning done wind doesn't change the flight plan distance (which is always measured relative to the Earth's surface), it changes your velocity. You could work it as a distance modification, but the math is much harder so we don't do that :)

It’s an interesting question. Most calculations that pilots do are based on fuel flow per hour. Several regulations include these types of calculations as well: you must have a fuel reserve for (say) 30 or 60 minutes beyond your final destination.
For some questions, eg comparing a small plane to a car in fuel economy, fuel per distance is interesting.
But, some other reasons why fuel per distance may not be as useful as fuel per hour:

Wind. A tailwind versus a headwind can dramatically change the fuel per mile, but doesn’t change the fuel per hour (if your airspeed is the same)
Comparison to other planes. If everyone compares figures per hour, then you have a solid basis for comparison. Of course, comparing a jet’s fuel per hour compared to a prop plane’s fuel per hour would be silly, but planes in the same class may have somewhat comparable cruising speeds
Taxi, climb and descent. The cruising speed of a plane might not be that relevant for short flights, when most of the time is not spent cruising
Pilot Operating Handbooks (POH)s tell you many figures in gallons per hour, including fuel usage while climbing, at different speeds, different atmosphere conditions etc

These are just a few. I guess neither fuel per hour nor fuel per distance is useful by itself—it depends entirely on what you want to compare.

As explained in other answers, fuel/time is better for planning, but now let's move to the cockpit!
Fuel/time is easily and accurately measured by the vehicle instrumentation and, as a plane throttle is mostly kept fixed, the value won't change much allowing you to use it to do estimations. A very convenient one is the residual flight-time: this tells you how much you can wait for landing and how much this wait will cost. From the fuel/time is not hard to get the range of autonomy once you know the ground speed.
Fuel/distance, instead, will make your life much harder if you need to know how much you can wait above an airport!
On the contrary, driving a car you want to know how far you can get to reach a gas station, not for how long you can drive around it, that's why fuel/distance is much better in this case.

For a given amount of fuel, the engines will only run for a certain amount of time. Distance is irrelevant. If you circle going nowhere for one hour, you've still consumed one hour's worth of fuel. If your airspeed is 100 knots and flying into a 100 knot headwind, you're still consuming fuel even though the distance traveled is 0.

If there's a 100-knot headwind, you probably shouldn't be flying.

Technically, the best metric would be fuel flow, per weight. For a simple answer, you burn far more fuel when you're heavier, than when you're lighter.

This is true, there could be a base fuel flow figure for a basically empty plane, and then additional fuel flow for each unit of weight carried. In reality, there are generally different fuel flow figures given for different weights, and other variables as well.


text:
Why has structural efficiency (OEW / MTOW) not improved despite increasing use of carbon fiber?
Where can I find information on the weight breakdown of civil aircraft? I am interested primarily in trying to understand why structural efficiency (OEW/MTOW) has not improved despite increasing use of carbon fiber materials.
I'm aware of the following questions:

Where to find information on weight breakdown for helicopters?
What materials make up most of the weight of an aircraft?
What is the weight of the fuselage (only) of the Boeing 747?
Are there specific mass breakdowns for aircraft?

Unfortunately, none of the answers list any authoritative sources. One links to an older MIT thesis, which includes a fractional weight breakdown of MD-80, DC-10-30, 737-200, 747-100, and A300-B2.

Well, simply consulting Wikipedia you can calculate that an "old" A330 has an empty weight fraction of (in tons) 132/251=0.545. A recent A350 has 142/283=0.50 i.e. some 10% lighter.
As a rule of thumb, a structural piece made of carbon fiber reinforced plastic is some 20% lighter in respect to aluminium. If you have a look at the figure 11 of the paper you quoted, you can see that only half of the empty weight is made up of structure (wing + fuselage + tailplane) the rest being systems, engines and landing gears.

So if you apply the rule of thumb of -20% to the 50% of empty weight composing the structure you end up with a 10% gain which is exactly what we have obtained comparing the previous two ratios for the A330 and the A350.

I've run into this problem in aircraft design classes - you don't have carbon fiber passengers, fuel, seats, or tires!

@costrom carbon fiber passengers is a good idea ;O)

Not entirely true that carbon fibre seats don't exist: https://www.compositesworld.com/articles/cfrp-upgrades-economy-seating

To add to sophit's correct answer, calculating structural efficiency as MTOW/OEW is not always informative. It is informative if both aircraft designs aimed to maximize this ratio.
For same weight, MTOW might not increase, if range and payload requirements have been satisfied, and there's no more space or need neither for fuel, nor for cargo.
Extreme examples would be the Cessna 172R with a 70% empty weight fraction, or the Convair B-58 with 31%.
With a better material, the aircraft designer can decrease empty weight, or they can invest the savings into a longer wingspan, better engines (higher bypass ratio for commercial, more power for GA), or something else entirely.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is boundary layer, exactly?
I read a lot about boundary layer but I don't know exactly what boundary layer is yet.

Welcome to aviation.SE! This question might be what you're looking for

Airflow (or any fluid flow) will be zero at the surface of the object it's passing over. Imagine the airflow as layers (or "streamlines"), and remember the surface layer velocity is zero. As you move away from the surface, each layer will be going a little faster than the one beneath it until you finally reach "free stream" velocity. All of these low-energy (lower velocity) layers of air between the aircraft surface and the free-stream air make up the boundary layer.
Here is an illustration from NASA:
(NASA)

The boundary layer is the part of the flow near the surface of a body where friction slows down the local flow. Directly at the wall of the body, flow speed is zero and increases the more you move away from that body. The boundary layer ends by definition when the flow has reached 99% of the speed of the outside flow.
An ideal fluid only transmits pressure forces. In reality, air is close to an ideal fluid but not quite, also transmitting shear forces. Those shear forces equalize the flow speed wherever there are speed gradients perpendicular to the flow direction, especially slowing down the flow near the wall. Note that streamwise speed gradients are caused by pressure forces.
At the stagnation point (where the flow hits the body for the first time) the boundary layer thickness is zero, but grows quickly downstream from there. A typical boundary layer thickness at the trailing edge of an airplane's wing is several centimeters.

Growth of a boundary layer on a flat plate (picture source). $v_e$ is the speed of the outside flow (well, 99% of it at the solid blue line), $u$ is the local flow speed and $\tau$ is the local shear stress.
Aerodynamics deals mostly with speed boundary layers, but there are also heat boundary layers, especially in supersonic flow.